In the recent trend of smaller size and higher density of electronic devices, there is an increasing demand for multi-layer structure of circuit boards in the fields of industrial and consumer products. In such circuit boards, it is required to develop newly a manufacturing method for connecting circuit patterns of a plurality of layers with inner via holes, and a circuit board structure having a high reliability. As the manufacturing method of the two-sided circuit board, a manufacturing method of circuit board connecting inner via holes with conductive paste is proposed. This conventional manufacturing method of circuit board is explained below.
FIG. 8 is a sectional view of mask film used in manufacture of conventional circuit board. FIG. 9 is a perspective view of a mask film in manufacture of conventional circuit board, showing the position of a large tension.
A conventional mask film 22 has a base material 11, and a parting layer 12 placed on the whole surface of the base material 11. The base material 11 is made of polyethylene terephthalate. In the manufacturing method of two-sided circuit board, the mask film is adhered to both sides of a prepreg sheet so that the parting layer 12 side may be positioned on the surface of the prepreg sheet. The prepreg sheet has a base and a resin impregnated in the base.
Penetration holes are formed in specified positions of the prepreg sheet to which the mask film 22 is adhered.
The penetration holes are filled with conductive paste. The conductive paste filling method comprises a step of putting the prepreg sheet having penetration holes on a table of a general printing press, and a step of applying conductive paste directly from above the mask film 22 by using a squeegee. At this time, the mask film 22 plays the role as printing mask, and also the role of prevention of contamination of the surface of the prepreg sheet.
The mask film 22 is parted from both sides of the prepreg sheet.
A metal foil is overlaid on both sides of the prepreg sheet.
The prepreg sheet having the metal foils is heated and pressed. In this process, the prepreg sheet and metal foils are mutually adhered. In this case, the metal foils adhered to both sides of the prepreg sheet are electrically connected to the conductive paste filling up the penetration holes formed in specified positions.
The both metal foils are selectively etched, and a circuit patterns are formed. Thus, a two-sided circuit board is manufactured.
In this conventional manufacturing method of multilayer board, however, when the prepreg sheet and mask film are adhered, as shown in FIG. 9, the shrinkage stress 30 at four corners of a prepreg sheet 31 is larger than at other positions. Peeling occurs from these four corners. In the subsequent manufacturing process of conveying and handling, peeling expands gradually.
By forming the parting layer 12 of the mask film 22 very thinly to be 0.01 μm or less, pin holes are intentionally formed in the parting layer 12. By these pin holes, the adhesion strength of the prepreg sheet and mask film is maintained.
When forming penetration holes in specified position by heating process such as laser process (in particular, CO2 laser), the polyethylene terephthalate of the mask film 22 is melted by the process heat, and the molten material is fused with components of the prepreg sheet 31. Accordingly, when parting the mask film 22 from the prepreg sheet 31, the resin component or base in the prepreg sheet 31 may be peeled together with the mask film 22.
Depending on presence or absence of penetration holes, the adhesion strength of the metal foils and the prepreg sheet 31 varies, and peeling is likely to occur by impact. Accordingly, part of the conductive paste may be peeled off. In this conventional method, conditions of these steps have large effects on the quality of the two-sided circuit board.